| For someone who is reasonably fit, I don't believe there will be any difference. As stated earlier, urge to breathe is dependant upon CO2 levels. However, for anyone who has breathing or circulatory difficulties, a richer mix allows that person to get the O2 they require with a normal breathing pattern - ie not gasping away like a steam train. They use less gass when breathing 100% for example, but I can't see how this is relevant to anybody who dives.
Freedivers purge CO2 in order to delay the point at which the CO2 levels compell breathing to restart. They need to use 100% otherwise they would become victims of shallow water blackout on their ascent.
Cheers, CHris
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